Type-2 NF1 deletions spanning 1.2 Mb are frequently of postzygotic origin and hence tend to be associated with mosaicism for normal cells and those harboring the deletion (del(+/−) cells). Eleven patients with mosaic type-2 deletions were investigated by FISH and high proportions (94–99%) of del(+/−) cells were detected both in whole blood and in isolated CD3+, CD14+, CD15+, and CD19+ leukocytes. Significantly lower proportions of del(+/−) cells (24-82%) were however noted in urine-derived epithelial cells. A patient harboring an atypical large NF1 deletion with nonrecurrent breakpoints was also found to have a much higher proportion of del(+/−) cells in blood (96%) than in urine (51%). The tissue-specific differences in the proportions of del(+/−) cells as well as the X chromosome inactivation (XCI) patterns observed in these mosaic patients suggest that the majority of the deletions had occurred before or during the preimplantation blastocyst stage before the onset of XCI. We postulate that hematopoietic del(+/−) stem cells present at an early developmental stage are characterized by a selective growth advantage over normal cells lacking the deletion, leading to a high proportion of del(+/−) cells in peripheral blood from the affected patients. Hum Mutat 33:541–550, 2012. © 2011 Wiley Periodicals, Inc.